Color vision of human eye starts to proceed with the stimulation of photoreceptors (also termed as cones), which can be found on the retina and are sensitive in three different wavelength regions, i.e. in the long (I) middle (m) and short (5) regions (see Stockman, A., Sharper L. T.: “The Spectral Sensitivities of the Middle and Long Wavelength Sensitive Cones Derived from Measurements in Observers of Known Genotype”; Vision Research 40, 1711-1739 (2000)). The process continues by forwarding the signals of these cones first into bipolar and then into ganglion cells (Rodiech, R. W: “The First Steps in Seeing”, p. 38-40 (Sinaver Associates, Inc., Massachusetts, USA, 1998)). The signals exiting the ganglion cells transport the chromatic information to the brain, where the further steps of color vision proceed.
The spectral sensitivities of the photoreceptors on the retina playa basic role in the appearance of parachromatism. Early attempts to correct parachromatism were based on the assumption that with color deficients some of the receptors are simply less sensitive than the normal ones, thus the sensitivities of the other receptors (with normal sensitivity) were lowered proportionally in order to try to recreate the correct ratios between the sensitivities of all of the receptors. Methods and means disclosed in published Hungarian patent application No. P9800510, in Canadian Patent No. 5,574,517 and in U.S. Pat. Nos. 4,998,817, 5,574,517, 5,617,154, 5,369,453 and 5,846,457, furthermore the use of color filters which selectively transmit less light in the wavelength region where a specified receptor is sensitive can be regarded as such attempts for correction.
Some other methods based on the use of color filters aimed only at causing a certain increase in color contrast, without attempting to attain a real improvement of color vision (see e.g. the method disclosed in U.S. Pat. No. 6,089,712). Such color filters frequently even deteriorate the vision of colors, they enable, however, color deficients to see different colors in different brightnesses. With such filters the so-called pseudoisochromatic color vision tests (such as Ishihara, Velhagen and Dvorintests) can well be “decepted”, without arriving at a real improvement in color vision.
Further investigations have shown that the basic reason for congenital color deficiency is that the spectral sensitivity functions (further on: SSF) of certain photoreceptors on the retina are shifted along the wavelength axis (Nathans, J.: The Genes for Color Vision; Scientific American pp. 35-38 {February, 1989)). This recognition well explains why the theory of decreased receptor sensitivity, which proved to be erroneous, could still remain for a prolonged period of time: namely if receptor sensitivity is measured only at a given wavelength, it may well occur that, just due to the shift, a part of SSF with lower sensitivity is observed.
The method disclosed in U.S. Pat. No. 5,774,202 makes use of the above recognition, wherein it has been also taken into account that the receptor SSFs may also be of incorrect shape, i.e. color deficiency is a result of SSFs with shifted position and/or of deformed shape. The principle of the method is that a color filter should be used, the spectral transmission function (a function where the percentage of transmitted light is plotted against the wavelength of the light for a given receptor type) of which corresponds to the quotient of SSF with correct shape and position to be attained and SSF with real (shifted) position and/or of deformed shape. This solution works well until the color filter has no adverse effects on other receptors with SSFs not to be corrected. This conflict is resolved so that the light transmission of the color filter is adjusted to the function calculated as discussed above only over a certain wavelength range, and the light transmission of the filter is maintained at a constant value at wavelengths outside of this range. The weak point of this solution resides just in this latter condition, since it is rather arbitrary to select the wavelength from which light transmission must remain unchanged.
All of the solutions discussed above intervene in the process of color vision from the side of receptors, i.e. from the input side. Although it has already been known (see e.g. pages 350-353 of Rodiech's textbook cited above) that a group of ganglion cells compares the signals of the I and m receptors and, as a result, transmits to the brain opponent signals which are proportional to (l−m), whereas a further group of ganglion cells compares the signals of the s receptors with the sum of l and m signals and, as a result, transmits to the brain opponent signals which are proportional to (s−(l+m)), no attempt was made to influence the intensity changes of the opponent signals as a function of the wavelength; i.e. feedback control from output side has not been applied before to correct color vision.